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Global Cycle Analysis of N2O Using Isotopomers Sakae TOYODA, Tokyo Institute of Technology 1. Introduction Nitrous oxide (N2O) is one of the greenhouse gases in the troposphere (IPCC, 2007) and is the most important ozone-depleting gas in the stratosphere (Ravishankara et al., 2009). Its global average tropospheric concentration in 2010 is about 322 ppb, which is lower than that of carbon dioxide (CO2) by three orders of magnitude. However, it has about 300 times greater global warming potential than CO2 over a 100-year time scale, and is increasing at the rate of about 0.7 ppb yr-1. Sources of N2O include natural and agricultural soils, aqueous environment such as oceans, rivers, and lakes, industrial processes like fossil fuel combustion, biomass burning, animal and human wastes (IPCC, 2007). About 90% of these sources are related to microbiological processes such as nitrification and denitrification, which occur naturally and can be enhanced in soils and waters that are enriched in nitrogen species due to human activity. In spite of a number of studies based on concentration analysis, there is still great uncertainty about the estimated magnitudes of global N2O sources. This is mainly because microbial activity in soils or waters is sensitive to environmental factors and thus production of N2O is not occurring uniformly with respect to space and time. In addition, it is difficult to identify the microbial pathway that mainly contributes to the N2O production in each study site. 2. Stable isotopes as a tool to investigate global cycle of N2O Although natural abundances of stable isotopes are almost constant (e.g., 14 N and 15 N respectively account for 99.64% and 0.36% of elemental nitrogen), they actually show a very small change during physical or chemical processes because of small difference in their bond energy or kinetic energy (isotope effect). Such a small variation is expressed by relative difference in isotope ratio (e.g., 15 N/14N) between sample and reference material. In chemical reactions, isotope ratio (or isotopomer ratios, when molecular species are considered) are determined by isotope (isotopomer) ratios in precursors and isotope fractionation factor that is specific to each reaction. Therefore, isotope/isotopomer ratios provide qualitative information that complements the quantitative information obtained through mixing ratio analyses. We developed a mass spectrometric method to measure the isotopomer ratios of N2O with high sensitivity and high precision in order to resolve the complex N2O cycle and to provide a basis for reduction of anthropogenic N2O emission using isotopes as an alternative tool (Yoshida & Toyoda, 2000). 3. Isotopic observations of N2O in various environment We have been conducting isotopic observations of N2O in the atmosphere, ocean, land waters, soils, and other sources. Monthly monitoring of surface air at Hateruma Island, Japan, showed that 15N/14N ratio in atmospheric N2O has been decreasing since 1999. In the western North Pacific, dissolved 2012 Japanese-American Kavli Frontiers of Science Ocean Acidification / N2O Page - 45 N2O showed a concentration maximum at about 500 m depth, and estimated 15 N/14N ratio and 18 O/16O ratio of N2O emitted from the ocean to the atmosphere are slightly larger than the values of atmospheric N2O. In the eastern tropical North Pacific, where biological productivity is high because of upwelling, the isotope ratios showed larger values suggesting that N2O is not only produced but also consumed (reduced) by denitrifying bacteria. Nitrous oxide emitted from fertilized agricultural soils showed significantly low the 15 N/14N ratio reflecting 15 N/14N ratio of nitrogen fertilizer and microbiological isotope effect. From the characteristic isotope ratios in N2O dissolved in river water in Tokyo, we found that significant amount of N2O is emitted from wastewater treatment plant (WWTP). Detailed studies on N2O produced in WWTP together with laboratory studies on pure culture of nitrogen-metabolizing bacteria revealed that the N2O emission depends on type of biological treatments and that isotopomer ratios can be used to specify the bacterial process that mainly contributes to the N2O production. 4. Analysis of global budget Based on above-mentioned observational data including those reported by other researchers, we analyze the global N2O budget using a mass balance of light and heavy isotopes between various sources, atmosphere, ocean, and removal processes. The isotope budget calculation with 14 N-containing N2O suggested that the isotopically light (i.e., 15 15 N- and 14 N/ N ratio is low) sources such as agriculture and industry contribute to the increase of atmospheric N2O and that the contributing source might have become heavier in recent years probably because of qualitative change in anthropogenic sources. Conclusion Stable isotopes have been used to trace the cycle of water, carbon dioxide, methane, and so on. Our study indicated that they can also be an invaluable tool for analysis of N2O cycle. Further studies on temporal and spatial distributions of N2O isotopomers and isotopic characterization of various N2O sources would enable (1) the global three-dimensional model simulations to understand the precise picture of N2O cycle and (2) proposals of how to reduce N2O emission by human activities. References IPCC (Intergovernmental Panel on Climate Change) (2007), Climate Change 2007: The Physical Science Basis., edited by S. Solomon et al., pp. 996, Cambridge Univ. Press, Cambridge, U.K. Ravishankara, A. R., J. S. Daniel, and R. W. Portmann (2009), Nitrous Oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century, Science, 326, 123-125. Yoshida, N., and S. Toyoda (2000), Constraining the atmospheric N2O budget from intramolecular site preference in N2O isotopomers, Nature, 405, 330-334. 2012 Japanese-American Kavli Frontiers of Science Ocean Acidification / N2O Page - 46